A warning has been issued that the habit of sleeping with a smartphone plugged into a charger and left under a pillow or blanket could increase the risk of overheating and fire. It is more important to safely vent heat generated during charging than to worry that mobile phone radio waves cause cancer.

According to tech outlet Engadget on Nov. 8, lithium-ion batteries widely used in smartphones generate heat during charging. If the device is left under a pillow or blanket, airflow is blocked, heat cannot dissipate properly and internal temperatures can rise.

If such overheating is repeated, battery performance can quickly deteriorate and internal components can be damaged. If temperatures become too high, the device's safety protection functions may activate and stop charging or operation.

If heat continues to build up in a damaged battery, a thermal runaway phenomenon in which internal heating cannot be controlled can occur. In such cases, there is a risk it could lead to fire or burns. The risk can be even greater if a charger that does not meet safety standards or a damaged cable is used.

Manufacturers also advise users not to charge devices under bedding. Apple instructs users to place an iPhone, charger and power adapter in a well-ventilated place and not to put or sleep on a device connected to power under a pillow or blanket.

On the other hand, research findings have said there is insufficient evidence to support an increased risk regarding a link between mobile phone radio waves and cancer incidence.

A systematic review commissioned by the World Health Organization and published in the international academic journal Environment International in 2024 assessed that exposure to radiofrequency electromagnetic fields from mobile phones is unlikely to increase the risk of several tumours in adults, including glioma, meningioma and acoustic neuroma.

Radiofrequency waves emitted by mobile phones are non-ionising electromagnetic waves with low energy, unlike X-rays or gamma rays. The study did not separately verify the act itself of sleeping with a smartphone under a pillow.

Experts recommend placing a smartphone on a hard, flat surface such as a bedside table or desk with good ventilation, rather than on a bed, for safe charging. They also say it is important to check chargers and cables for damage and to ensure the device is not covered by bedding while sleeping.